Negative feedback circuit



June 16, 1942. BRUCK 2,286,337

NEGATIVE FEEDBACK CIRCUIT Filed Nov. 18, 1938 NEGATIVE FEEDBACK TOS/GNAL 7. SOURCE /VEGA TIVE FEEDBACK I NV EN TOR.

ZHAR BRU'cK BY I ATTORNEY.

Patented June 16, 1942 assignor to Telefunken Gesellschaft fur Drahtlose Telegraphic m. b. 11., Berlin, Germany, a

corporationof Germany :Applicanbanovember 18, 1938, Serial Nd. 241,122

: In Germany November 2'7, 1937 icia'i (01. 250-20 For the purpose of reducing non-linear harmonic distortion in amplifiers it has been suggested in the prior art to apply the amplified alternating potential to the input end of the amplifier, or part of the amplifier, by negative feedback or degeneration. Such negative feedback may be brought to act either upon the cathode or a control grid of an amplifier tube. When the reverse feedback relationship is established with the cathode, the latter is at alternating potential, and this may be undesirable occasionally. The present invention is concerned with the case where negative feedback is estab lished with the control grid of an amplifier tube; that is, the same grid upon which the alternating potential to be amplified is impressed. According to the invention, the voltage to be amplified is impressed upon the control grid acros a resistance, whereas the voltage in reverse feedback is applied directly to the control grid while the said resistance is circumvented, or bypassed.

The advantage of the invention reside in the fact that the voltage returned to the control grid is in phase opposition to the impressed voltage. As a result, part of this potential will be compensated, and as a consequence the ap parent input resistance, that is to say the alternating current resistance between the control grid and the cathode, is apparently reduced.

However, this input resistance is connected in parallel to the load resistance for the electromotive force furnishing the input alternating potential. In other words, the input potential applied to the load resistance, and thus the eifective load or ohmic resistance, also are diminished. However, this is undesirable in many cases. For instance, if the alternating potential fed to the control grid is taken off at the plat of a prior triode, the negative feedback due to the reduction of the outer resistance of the triode means an increase in the non-linear harmonic distortion, also called the blur factor of this preceding or input tube. The situation is similar if the amplifier is connected, say, with the load resistance of the receiver diode of a receiver. A resistance included in the control grid lead, if of suitable value, decouples the input alternating voltage source from the inverse feedback potential so that the load resistance is not essentially reduced by such negative feedback.

An exemplified embodiment of the invention, in which the fundamentals thereof are incorporated, is shown in Fig. 1. Fig. 2 shows a modification. The source of alternating potential supply E in Fig. 1 Works across the inner resistconnected with each other.

ance R1 upon the outer load resistance R; whence the potential i taken off, and through the blocking condenser C and resistance R is impressed upon the control grid of the amplifier V of the triode type. From the output end of the amplifier (optionally from the very plate of the tube V), part of the amplified alternating potential is taken off, and this fraction, in reversed phase, by way of the blocking condenser C1 and the ohmic resistance R1 is impressed upon the control grid of tube V.

In the circuits of this kind usually adopted in the past, the resistance R was absent; in other words, the two points 111 and 102 were directly The potential brought back by negative feedback was also in alternating current connection with the load resistance Ra, and thus occasioned an apparent reduction of the entire outer resistance of the voltage source E. But, by including the resistance R conditions can be made so that the voltage in phase opposition between P2 and ground is divided between point P1 and ground in such proportion that the major portion of the potential in reverse feedback arises across the resistance R, while only a small portion arises at point 501. In order that this condition may be secured as perfectly as possible it is necessary that resistance R should be substantially higher than a resistance resulting from a parallel connection of R1, Ra and the grid leak R If resistance R be proportioned as stated, then also the elements of the return branch may be chosen so that the output of the amplifier will be less loaded by the inverse feedback circuit. In other words, R1 may be chosen to have a higher value. If the grid leak Rg is sufiiciently high, in other words substantially greater than R, then its upper end may be connected with point n rather than with point P1; that is, directly with the grid.

In the circuit of Fig. 2 the input alternating voltage source is represented by the load resistance Ra of the receiver diode D. The nongrounded end of this load resistance is connected through condenser C and resistance R with the control grid of the amplifier tube V1 whose plate is connected with the control grid of the second amplifier tube V2. The output potential of this amplifier, by way of the transformer T, is fed to the loudspeaker L. One end of the secondary coil T1 of the transformer is grounded, whereas the other end is connected through resistance R1 with the control grid of tube V1, the cathode potential of which, as usual,

is positive to ground. The polarity of the secondary coil must be so chosen that the potential negatively fed back through R1 to the grid of tube V1 is impressed upon the control grid of tube V1 in phase opposition to the rectified input, or signal, voltage. The amplitude of the reverse feedback potential, and thus also the degree of the negative feedback, may be adjusted by the value of R1. The filter comprising the resistance R and the grid-filament capacitance of tube V1 serves at the same time for filtering out such radio frequency oscillations as still arise across th resistance Ra.

What is claimed is:

In combination with a demodulator of audio modulated carrier waves provided with a load resistor, an audio amplifier tube including at least a signal grid, cathode and plate, a second resistor connecting the high potential end of said load resistor to said grid, a grid biasing resistor in the space current path of said amplifier tube between said cathode and ground, an audio output circuit coupled to said amplifier plate and including an audio output transformer having its secondary in circuit with the voice coil of a loud speaker, one end of the transformer secondary being at ground potential, a degenerative audio voltage feedback connection between the opposite end of said secondary and the junction of said grid and said second resistor, said feedback connection being the sole conductive path from the signal grid to ground whereby said feedback path acts additionally as the sole bias path to apply the direct current voltage developed across said biasing resistor to said grid, the resistance value of said second resistor exceeding that of the demodulator and the load resistor, and said second resistor cooperating with the inherent grid to cathode capacity of the amplifier tube to provide a filter of carrier fre- 20 quency currents. 

